1. Field of the Invention
The present invention relates to the field of clinical diagnostic reagents and more particularly to reagents containing a hormone substance tagged with radioisotopic iodine and useful for in vitro (thyroid) function determinations.
2. Description of the Prior Art
It is known to measure the unsaturated binding capacity of blood serum by admixing a sample of serum with a buffered solution containing a known amount of thyroid hormone labeled with a radioactive isotope of iodine and an ion-exchange resin or other solid sorbent substance such as charcoal, talc, magnesium silicate, and the like capable of binding the excess labeled hormone so that it can be separated and removed from the serum sample. The amount of labeled hormone taken up by the serum protein, primarily thyroxine-binding globulin, can then be determined directly or indirectly using a scintillation counter. Such tests are commonly known in the art as T-3 tests.
In the most commonly used forms of this test, the solid sorbent material is added after the serum sample and the labeled hormone solution are combined in which case the function of the sorbent is limited to binding the excess, i.e. free or unbound, hormone so that it can be separated from the liquid serum sample containing the protein-bound hormone.
In another form of this test (U.S. Pat. No. 3,507,618), the labeled hormone is initially bound to an ion-exchange resin, and in this form the test depends upon desorption of the hormone from the ion-exchange resin carrier. This method is satisfactory provided that the charged resin is freshly prepared and is kept thoroughly dry up until the moment of actual use, and provided that all times and temperatures are carefully standardized and precisely controlled.
A major defect with this method is that with time, and especially in the presence of moisture, the labeled hormone becomes permanently bound to the resin and therefore unavailable for binding by the blood serum protein. The combination of a labeled hormone and an exchange resin is therefore entirely unsuitable for use in a complete and self-contained liquid reagent composition that can be prepared and stored for future use.
Eisentraut U.S. Pat. No. 3,666,854 discloses a method for determining thyroid function in which particulate inorganic crystalline materials are employed as sorbent materials, and specifically discloses the use of silicic acid and opal, for example. However, we have found that amorphous silicon dioxide per se is not particularly effective when employed as the sorbent material in a test for determining thyroid function.